Blender won't import my custom module, ImportError: No module named mymodule
You can place your module into the folder blender_directory/2.68/scripts/modules/ (so Blender can found it automatically), while your addon can stay in the scripts/addons/foldername folder with its __init__.py (and not __index__.py, by the way).

Or, you can go to User Preferences, tab File, and specify a custom extra folder for scripts, in the scripts input.
Note that this custom folder must contain 3 sub-directories : addons, modules, and startup.



In this example, your addon will of course be placed in :
/home/user/scripts/blender/addons/yourAddon/,
and the module(s) you want to import will be into :
/home/user/scripts/blender/modules/.

This way, no need to use sys.path.append()!